This application claims the priority of German patent document 195 46 942.6 filed Dec. 15, 1995, the disclosure(s) of which is (are) expressly incorporated by reference herein.
The present invention relates to a hydraulic power steering system for motor vehicles which has a reversible hydrostatic servomotor connected (with respect to the drive) to mechanical steering elements of steered vehicle wheels. The invention also has a servo valve arrangement which can be controlled via a steering handle (or a manual steering wheel) for the controllable connection of the servomotor with a hydraulic pressure source or a relatively pressureless reservoir.
Power steering systems of this type are generally known and are installed into motor vehicles in series. The servo valve is typically controlled by parts of a mechanical drive connection (or a steering column) between the manual steering wheel and the steering elements. For this purpose, parts of this drive arrangement can be adjusted, relative to one another, as a function of the transmitted forces and torques. The respective adjusting movement is transmitted to the control parts of the servo valve.
Particularly from the field of aircraft manufacturing, it is known to eliminate a mechanical through-drive between the steering handle and the steering elements (for example, pivoted flaps as well as horizontal tail units and rudder units), and to couple the steering handle with the steering elements only with respect to the effect via a control system. In this case, the steering handle operates a desired-value generator and the control system operates a servomotor of the respective steering element which corresponds to a desired-value/actual-value comparison. These steering systems, which are also called "fly-by-wire" systems, have achieved such a high reliability that they are also used in series in passenger planes. A special advantage of such systems is their low space requirement.
It is an object of the present invention to provide a new power steering system which is distinguished by a low space requirement while using as many tested and reliable system components as possible.
This and other objects and advantages are achieved by providing a steering handle which operates an electric or electronic desired-value generator. The electronic desired-value generator is connected with an input of an electronic control system which, in turn, is connected on the output side with the servo valve arrangement. The electronic desired-value generator is also connected via another input with an actual-value generator. This actual-value generator reflects the steering position of the steered vehicle wheels. It is possible to constantly and automatically check the control system with respect to a no-fault operation. Furthermore, the steering handle is forcibly coupled with a reversible hydrostatic delivery element. Additionally, a change-over valve unit is arranged between the servomotor and the delivery element and the servo valve. In the event of a control system malfunction, the change-over valve unit connects the servomotor (while forming a hydraulic linkage) with the delivery element and separates it from the servo valve. In the case of the control system functioning correctly, the change-over valve unit connects the servomotor to the servo valve, on the one hand, and the input and output of the delivery elements to each other, on the other hand.
The present invention is based on the general idea of controlling (during normal operation) the servo valve used for controlling the servomotor only by a desired-value/actual-value comparison of the steering position, and in an emergency (or particularly in the event of disturbances of the control system) establishing a forced hydraulic coupling between the steering handle and the steering elements of the vehicle wheels.
During the operation of the control system, a flexible steering characteristic can be provided in this manner. In particular, the transmission ratio between the steering angle change of the steered wheels and the adjusting stroke of the steering handle (or of the steering wheel) required for this purpose can be changed as a function of parameters.
Minimum space is required for the control system, as well as for the optionally connected hydraulic linkage. Furthermore, the ability to house the system components which are arranged between the steering handle and the steering elements of the steered wheels in a virtually arbitrary manner is particularly advantageous.
For increasing the steering precision of the hydraulic linkage in the event of a malfunctioning of the control system, a preferred embodiment of the invention provides that the delivery element of the steering handle is constantly supplied with a pressure medium (that is, is kept under a hydraulic prestress), and correspondingly constant complete filling with an hydraulic medium is ensured.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.